1540s, "to tread a path," from trade (n.). Meaning "to occupy oneself (in something)" is recorded from c. 1600. Meaning "to barter" is by 1793. The U.S. sports team sense of "to exchange one player for another" is attested from 1899. Related: Traded; trading. To trade down is attested from 1942; trade up from 1959. Trade places "exchange situations" is from 1917. Trading post is recorded from 1796. Trading stamp, given by merchants and exchangeable for goods, is from 1897.
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Definitions of trader from WordNet
someone who purchases and maintains an inventory of goods to be sold;